Cargo boxes now in use on tractor trailers are usually provided with a pair of rear doors swinging on vertical axis hinges and meeting at the mid-width of the end opening of the box. When the doors are swung fully open the access opening is virtually equal to the box cross-section. After the cargo has been loaded the doors are swung to the closed position and each is latched shut by imparting rotation to a vertical latch bar to cause upper and lower latch jaws to lock in keepers fixed in the door frame. After the doors are closed, no opening exists whereby the state of the cargo may be inspected, for example after a highway trip. It is not uncommon for the cargo to shift; for instance drums or cases may come loose and slide or roll against one or both doors. When a person standing behind the trailer releases the latches holding a door closed, that door may be forcibly driven outward by cargo pressing against it, with risk of serious injury to that person.
Although devices have heretofore been proposed to prevent one or both doors from swinging freely open when unlatched, such as chains connected to the floor and to the inner side of a door, it is undesirable to install anything that could interfere with free rolling of dollies or handling equipment. Moreover, in such arrangement, the person who later actuates the lever arm to unlatch a door cannot know for certain if the device had been properly installed before the door was sealed.
Other devices which have been proposed include chains or cables intended to be connected between the doors externally after they have been closed. While safety to the operator can be assured by properly installing slack links of this kind, instances can arise where they fail to be put on, either through not being available when the doors are sealed, or through human error or neglect.
Various forms of modified latch bar handles have been proposed, and various rotation-limiting stop devices have been patented to minimise risk of injury to an operator's head or hands by a rapidly moving handle. Such devices and apparatuses may be reviewed in Olander U.S. Pat. No. 2,316,359 (1943), Dath U.S. Pat. No. 2,472,146 (1949), Dath U.S. Pat. No. 2,567,366 (1951), Danielson U.S. Pat. No. 2,676,046 (1954), and Olander U.S. Pat. No. 3,329,456 (1967). While such devices are useful in limiting kinetic effects or in reducing handle torque, the overall safety of the operator has not be adequately addressed.